The problem is here in latest framework version. It works well with 2.0.15.1.

I've translated issue into English so community can participate as a whole.

If the version of MongoDB was not changed, then it seems the problem is somewhere in the framework. Since you have a clear way to reproduce it please do bisect to find out exact commit that broke it. Here's how: https://git-scm.com/docs/git-bisect or in Russian: https://git-scm.com/book/ru/v1/Инструменты-Git-Отладка-с-помощью-Git

This problem after commit:

• 8bb334b

Class yii\mongodb\Session extends MultiFieldSession

Function MultiFieldSession::composeFields was changed.
If you return the old feature code, it works fine.

@webspec2012 can you check if it's 8bb334b#r33018458?

The session ID changes from one page to another.

..

db.BackendSessions.remove({});
WriteResult({ "nRemoved" : 4 })

refresh site page by F5

db.BackendSessions.find().pretty();
{
"_id" : ObjectId("5d79f9882893100f3029bfa4"),
"id" : "3racnq6iq3gtm8ma3has4nueva",
"data" : "__flash|a:0:{}__id|s:24:"5a65db240a64100202b7ca65";"
}

refresh site page by F5

db.BackendSessions.find().pretty();
{
"_id" : ObjectId("5d79f9882893100f3029bfa4"),
"id" : "04gl55gs0ecj22q61tu3s9l475",
"data" : "__flash|a:0:{}__id|s:24:"5a65db240a64100202b7ca65";"
}

refresh site page by F5

db.BackendSessions.find().pretty();
{
"_id" : ObjectId("5d79f9882893100f3029bfa4"),
"id" : "ilcs8pkjetisbie47p0h524adb",
"data" : "__flash|a:0:{}__id|s:24:"5a65db240a64100202b7ca65";"
}

...

Does PHP write it into MongoDB at all?

This log for each page..

@lubosdz any idea on why would it break?

my 2 cents, this looks sketchy - I would start investigating that:

@ailmanki thanks, that would be awesome.

Mongodb Session class not extends DbSession.

Mongodb Session class not extends DbSession.

So is the issue solved ?

I reckon yii2-mongo sessionWrite has not been updated accordingly after bugfix - compare:
https://github.com/yiisoft/yii2-mongodb/blob/master/src/Session.php#L137
https://github.com/yiisoft/yii2/blob/master/framework/web/DbSession.php#L186

Unfortunatelly I dont have mongo setup ..

Edit:
Related mongo issue #279 - see e.g. failing build.

So is the issue solved ?

No.

Fixed in PR #285.

CI fail not related to this PR - see build log - PHP 5.5. not pre-installed (is PHP 5.5 officially supported or can this be removed?).

@webspec2012 can you test with dev-master of the yiisoft/yii2-mongo?

The `dev-master ' branch works correctly!